udpateconninfo: mark variable unused

When compiling without getpeername() or getsockname(), the sockfd
paramter to Curl_udpateconninfo() became unused after commit e91e4816
added ifdef guards.

Closes #3910
Fixes https://curl.haxx.se/dev/log.cgi?id=20190520172441-32196
Reviewed-by: Marcel Raad, Daniel Stenberg